I. Field of the Invention
The present invention relates to a field-effect transistor using a thin organic film for a gate portion.
II. Description of the Prior Art
Recent progress in a technique represented by the Langmuir-Blodgett process (to be referred to as the LB process hereinafter) for forming a super thin film of organic molecules resulted in active development in techniques for applying a thin organic film to various elements. Research in the development of such techniques is being done by many organizations, such as research by G. G. Roberts at Durham University of an MIS element using a thin organic film. However, an element having a novel function effectively utilizing the characteristics of a thin organic film has not yet been realized.
In terms of applications to an electronic element, the most significant characteristic of the organic material is a charge transfer phenomenon between molecules. Organic molecules include donor molecules which have a low ionization potential and supply electrons to other molecules to take on a positive ionic state, and acceptor molecules which have a high electronic affinity and receive electrons from other molecules to take on a negative ionic state. It is known that a compound called a charge transfer complex is formed between these two types of molecules. For example, a compound of perylene and tetracyanoquinodimethane (TCNQ) consists of neutral molecules in which no charge is transferred, but tetramethylphenylenediamine (TMPD) and TCNQ form a compound in which respective molecules are in positive or negative states. In addition, a compound of TCNQ with tetrathiofluvalene (TTF) is known to change from a neutral to an ionic state due to a change in temperature or pressure.
When such a charge transfer phenomenon of the organic material is applied as an operational principle of an electronic element, the charge transfer characteristics such as efficiency, response time, and controlability must be excellent, and a material and an electronic element causing charge transfer must be easy to prepare. In this case, as for a charge transfer complex, preparation of crystals is very difficult, and external control of charge transfer is also difficult. It has been proposed to control charge transfer between a metal film and an organic molecule film by light or an electric field for application to a switching or memory element. However, a big problem lies in the efficiency, response time, and lifetime of a charge transfer. Thus, although charge transfer may be applied to various elements, it has not yet been practically used.
On the other hand, development of a novel functional element which cannot be realized by a conventional MOSFET LSI using silicon is expected. For example, a research is being done on an electronic element using a super lattice structure consisting of a GaAs film and an AlGaAs film as a compound semiconductor. However, since lattice constants or lattice structures of two kinds of compounds must be matched to obtain a super lattice structure of a compound semiconductor, selection range of compounds is limited and fabrication of the super lattice is costly and time-consuming.
Furthermore, an FET using a compound semiconductor has various problems that a MOSFET using silicon never has. For example, because of a high interface state density, a surface potential of a semiconductor does not change substantially even when a gate voltage is applied. This is a so-called pinning phenomenon of the Fermi level. In addition, since mobility of carriers of GaAs or InP is higher than that of Si, a high speed operation can be expected. However, an oxide film with less defects, such as an SiO.sub.2 film in the case of silicon, cannot be obtained by such compound semiconductors. More specifically, an oxide film obtained by performing thermal oxidation or anodic oxidation of such a compound semiconductor has a very high interface state density. For this reason, when a MIS type FET is formed using as a gate insulating film an oxide film obtained by thermal oxidation or anodic oxidation or another inorganic oxide such as SiO.sub.2 or Al.sub.2 O.sub.3 obtained by, e.g., the CVD method, no FET operation occurs because the surface of the semiconductor is not inverted, due to a high interface state density.
As described above, although an electronic element having a novel function which cannot be obtained by a conventional MOSFET using silicon is expected, it has not yet been realized. In addition, because a good gate insulating film cannot be obtained and an interface state density is high in an FET using a compound semiconductor which is expected to have a higher performance than that of a MOSFET using silicon, an MIS type FET has not yet been realized.